The parameters of a function to be filtered

I am new in WordPress and I am doing my first theme with Thematic.
I have been using filters and hooks to customize my theme without problems,
however, there are some things I haven´t understand completely

I have a doubt undertanding when to use $content (or another parameter) inside the parenthesis of my function:

And I was wondering why there is no need of putting any argument inside the parenthesis of the function.

I would apreciatte if somebody could give me an answer or a link I can read about this topic: The parameter / argument of the function to be filtered ( In the examples I´ve seen, sometimes is $content, other times is another variable, and other times there is no variable)

you are new to WP and this is your first theme? your understanding of filters is a damned site better than mine was at the same time... to answer your question, you don't have to pass in an argument. you do that when you want to preserve the variable in its original state. then you can modify it in your function. if you are completely overwriting everything you don't need to keep it as the variable will be generated completely from your function.